As a surface treatment of metal substrates as zinc plate, galvanized sheet steel, aluminium plate and the like, has been widely used a technique wherein a metal surface is merely treated with the so-called coating-type, non-rinse chromate treating agent comprising an aqueous resin and a water-soluble chromium compound, because of its easy operation and operational control and reduced number of required prosessing steps. Particularly preferred treating bath contains, as main ingredients, an emulsion obtained by the polymerization of at least one .alpha., .beta.-ethylenically unsaturated monomer in the presence of water-soluble high molecular compound as an emulsifier, and a water-soluble chromium compound from the standpoint of attaining excellent anti-corrosion and coat adhesion properties of the formed coating.
However, in considering surface pre-treatment of a metal plate, it is of great importance to take into account, not only the stationary adhesion between the plate substrate and the composition, but also the coating adhesion under bending and processing conditions, and the scratch resistance of the coating. Generally speaking, coating adhesion under bending and scratch resistance are mutually conflicting properties, and therefore, heretofore proposed coating type, non-rinse chromate treating agents have failed to give the desired properties in full for metal plates.
Under the circumstances, the present inventors have made various proposals on the method for treating metal surfaces and compositions to be used therefor.
That is, in Japanese Patent Publication No. 39393/81, was provided a method for the treatment of a metal surface which comprises applying a treating liquid comprising an emulsion and a water-soluble chromium compound containing 30 to 90% by weight of hexavalent chromium and optional water-insoluble white carbon to a metal surface and drying the resultant, said emulsion being prepared by emulsion polymerization of .alpha., .beta.-monoethylenically unsaturated monomer in the presence of an emulsifier selected from the group consisting of a polyacrylic acid and a copolymer of acrylic acid and a monomer selected from methacrylic acid, an acrylamide, a methacrylamide and a hydrophilic monomer of the formula: ##STR1## wherein A is hydrogen atom or methyl; R is a substituted or unsubstituted alkylene having 2 to 4 carbon atoms; and X is a functional group containing at least one of oxygen atom, phosphorus atom and sulfur atom.
In Japanese Patent Application Kokai No. 213064/83, was provided an anticorrosive metal surface pretreating composition comprising an emulsion of polymer particles and a water soluble chromium compound, said particles being prepared by the emulsion polymerizatin of .alpha., .beta.-ethylenically unsaturated monomers in the presence of an emulsifier selected from the group consisting of a polyacrylic acid and a copolymer of acrylic acid and a monomer selected from methacrylic acid, an acrylamide, a methacrylamide and a hydrophilic monomer of the formula: ##STR2## wherein A is hydrogen atom or methyl; R is a substituted or unsubstituted alkylene having 2 to 4 carbon atoms; and X is a functional group containing at least one of oxygen atom, phosphorus atom and sulfur atom, and formed as hard particles through internal gelation (or crosslinking) by the selection of monomer having two or more ethylenic unsaturations as a part of said .alpha., .beta.-ethylenically unsaturated monomers or having a glass transition point from 15.degree. to 110.degree. C. by the selection of monomers used.
And, in Japanese Patent Application Kokai No. 197575/84 was provided an anticorrosive metal surface pretreating composition comprising an emulsion of polymer particles comprising at least one .alpha., .beta.-ethylenically unsaturated monomers bearing thereon, as protective colloid, an oligomer with hydrophylic groups comprising at least one .alpha., .beta.-ethylenically unsaturated monomers, the mean diameter of the particles being 0.1 to 3.mu. and the particles being formed as hard microparticles through internal gelation or having a glass transition point from 15.degree. to 110.degree. C. and a water soluble chromium compound.
These compositions, when applied on metal plates, could give coatings with excellent coating adhesion under bending and scratch resistance, as well as excellent corrosion resistance, humidity resistance and the like and therefore, the thus coated metal plates and especially hot dip galvanized sheet steel have been well accepted as exterior metal panels to be formed for the production of such products as roof materials, wall panels, weather doors, shutters and the like. However, for the coating of hot dip galvanized sheet steel to be processed under more severe conditions, for the production of implement, houshold appliance and the like, e.g. low deposit (180g/m.sup.2 or less) hot dip galvanized sheet steel and low lead galvanized sheet steel, the coating vehicle should be of more flexible nature which in turn, causes lowering in the desired scratch resistance of the coating.
Therefore, a further development has been demanded in giving a metal surface treating composition which can be used in that particular field and result an excellent precoated metal plate for heavy forming use, with good coating adhesion under bending and scratch resistance, as well as corrosion resistance and other desired properties. The invention has been made to solve the abovementioned questions.